Bearing part and rolling bearing

ABSTRACT

Each of an outer race, an inner race and a ball as a bearing part is a bearing part made of JIS standard SUJ2 and having a carbonitrided layer formed in a surface thereof, wherein, after heat treatment at a heating temperature of 500° C. for a retention time of one hour is performed, a Vickers hardness at a position with a depth of 0.02×T+0.175 (mm) from the surface is higher than a Vickers hardness at a core portion, which is a region where the carbonitrided layer is not formed in a thickness direction of the bearing part, by not less than 80 HV, where T represents the time of carbonitriding treatment for forming the carbonitrided layer.

TECHNICAL FIELD

The present invention relates to a bearing part and a rolling bearing,and more particularly to a bearing part having a carbonitrided layerformed in a surface thereof, and a rolling bearing including the bearingpart.

BACKGROUND ART

It has been conventionally qualitatively known that carbonitridingtreatment is effective to extend rolling contact fatigue life of arolling bearing (see, for example, Japanese Patent Laying-Open No.6-341441 (PTD 1) and “Rolling Fatigue Characteristics of Carburized orCarbonitrided 1% Cr Steel at Elevated Temperatures” by Hyojiro Kurabe etal., Iron and Steel, vol. 11, (1967), pp. 1305-1308 (NPD 1)). Further,in recent years, it has become clear that carbonitriding treatment isalso effective in a method for evaluating dent-originated peeling lifeof a rolling bearing, which is a more quantitative method (see, forexample, Japanese Patent Laying-Open No. 2009-229288 (PTD 2)). It hasalso become clear that, if a nitrogen concentration in a productuppermost surface is not less than 0.1 mass %, the life is extended tothe extent that its statistically significant difference from that of anon-nitrided part is reliably recognized. In addition, it has also beenfound that, if the nitrogen concentration in the product uppermostsurface (raceway surface or rolling surface) is assured to be not lessthan 0.4 mass %, the life is further extended as compared with the caseof not less than 0.1 mass % (see, for example, “Estimation of ScratchedContact Fatigue Life with Artificial Dent of SUJ2 Steel Carbonitrided toControlled Surface Nitrogen Content” by Chikara Ohki et al., Iron andSteel, vol. 95, (2009), pp. 695-703 (NPD 2)). Accordingly, if it ispossible to provide a component of a rolling bearing which is assured tohave a nitrogen concentration of not less than 0.4 mass % in a productuppermost surface, safety of a rolling bearing is assured more reliably,providing a great social advantage as a result.

At present, however, a nitrogen concentration in steel can be quantifiedonly by a method performed using analysis equipment such as an EPMA(Electron Probe Micro Analyzer) and a GDS (Glow Discharge Spectrometer),and such a method requires a great number of man-hours. Therefore, ifsuch a method is adopted as a method for quality assurance, the rollingbearing becomes very expensive, which is not practical.

In addition, a method for qualitatively evaluating the degree ofnitridation by utilizing the characteristics that, when nitrogen hasentered steel, the hardness after high-temperature tempering becomeshigher than that of a non-nitrided portion is described (see, forexample, “Effects of Nitrogen Content on Microstructure and Resistanceto Softening during Tempering of Carbo-Nitrided Chromium Alloy Steels”by Youichi Watanabe et al., Heat Treatment, vol. 40, (2000), pp. 18-24(NPD 3)). Furthermore, there has also been proposed a method forproviding a component of a rolling bearing which is assured to have anitrogen concentration of not less than 0.1 mass % in a productuppermost surface, by using the aforementioned characteristics tomeasure a cross sectional hardness distribution after high-temperaturetempering (see, for example, Japanese Patent Laying-Open No. 2011-209021(PTD 3)).

CITATION LIST Patent Document

-   PTD 1: Japanese Patent Laying-Open No. 6-341441-   PTD 2: Japanese Patent Laying-Open No. 2009-229288-   PTD 3: Japanese Patent Laying-Open No. 2011-209021

Non Patent Document

-   NPD 1: “Rolling Fatigue Characteristics of Carburized or    Carbonitrided 1% Cr Steel at Elevated Temperatures” by Hyojiro    Kurabe et al., Iron and Steel, vol. 11, (1967), pp. 1305-1308-   NPD 2: “Estimation of Scratched Contact Fatigue Life with Artificial    Dent of SUJ2 Steel Carbonitrided to Controlled Surface Nitrogen    Content” by Chikara Ohki et al., Iron and Steel, vol. 95, (2009),    pp. 695-703-   NPD 3: “Effects of Nitrogen Content on Microstructure and Resistance    to Softening during Tempering of Carbo-Nitrided Chromium Alloy    Steels” by Youichi Watanabe et al., Heat Treatment, vol. 40, (2000),    pp. 18-24

SUMMARY OF INVENTION Technical Problem

According to the method proposed in PTD 3 described above, it ispossible to assure that the nitrogen concentration in the productuppermost surface is not less than 0.1 mass %, whereas it is difficultto assure a higher nitrogen concentration. Therefore, from theperspective of assuring a higher degree of safety in the rollingbearing, there is a demand for providing a bearing part which is assuredto have a higher nitrogen concentration.

The present invention has been made in view of the aforementionedproblem, and an object of the present invention is to provide a bearingpart which is quantitatively assured to have a higher nitrogenconcentration than that of a conventional one, and a rolling bearingincluding the bearing part.

Solution to Problem

A bearing part in accordance with one aspect of the present invention isa bearing part made of JIS standard SUJ2 and having a carbonitridedlayer formed in a surface thereof. In the bearing part, after heattreatment at a heating temperature of 500° C. for a retention time ofone hour is performed, a Vickers hardness at a position with a depth of0.02×T+0.175 (mm) from the surface is higher than a Vickers hardness ata core portion, which is a region where the carbonitrided layer is notformed in a thickness direction of the bearing part, by not less than 80HV, where T represents the time of carbonitriding treatment for formingthe carbonitrided layer.

A conventional bearing part can assure that a nitrogen concentration ina surface is 0.1 mass %, by utilizing the fact that there is acorrelation between a Vickers hardness and a nitrogen concentration inthe carbonitrided layer subjected to the aforementioned heat treatment(at a heating temperature of 500° C. for a retention time of one hour),and that nitrogen concentration distribution is shifted toward an innerside in the thickness direction by a predetermined distance (0.03 mm)due to the aforementioned heat treatment. Namely, the Vickers hardnessat the position with the depth of 0.03 (mm) from the surface subjectedto heat treatment as described above is improved as compared with thatat the core portion which is a region where the carbonitrided layer isnot formed, by a predetermined value, in accordance with the nitrogenconcentration at the position (i.e., the nitrogen concentration in thesurface before the aforementioned heat treatment is performed).Therefore, by setting a reference value for an improvement amount of theVickers hardness caused by the aforementioned heat treatment beforehandin accordance with a designed nitrogen concentration in the surface ofthe bearing part before the aforementioned heat treatment is performed,and determining whether or not a difference between a measurement valueof the Vickers hardness at the position with the depth of 0.03 (mm) fromthe surface subjected to the aforementioned heat treatment and ameasurement value of the Vickers hardness at the core portion satisfiesthe reference value (i.e., whether or not the difference is more thanthe reference value), whether or not the nitrogen concentration in thesurface of the bearing part is not less than the designed nitrogenconcentration (0.1 mass %) can be determined.

However, when the nitrogen concentration is in a range of not more than0.1 mass %, the aforementioned correlation between the Vickers hardnessand the nitrogen concentration in the carbonitrided layer is strong,whereas when the nitrogen concentration is in a range of more than 0.1mass %, the correlation is relatively weak. Therefore, in theconventional bearing part, it has been difficult to assure that thenitrogen concentration in the surface is not less than 0.4 mass %.

Thus, the inventor of the present invention has arrived at the presentinvention by utilizing the fact that a distance between a depth positionwith a nitrogen concentration of 0.06 mass % and a depth position with anitrogen concentration of 0.4 mass % is represented by 0.02×T+0.145 (mm)(T: the time of the carbonitriding treatment), in addition to the factthat the aforementioned correlation is strong when the nitrogenconcentration is in the range of not more than 0.1 mass % and that thenitrogen concentration distribution is shifted in the thicknessdirection by 0.03 (mm) due to the aforementioned heat treatment. Basedon the aforementioned correlation, the improvement amount of the Vickershardness corresponding to the nitrogen concentration of 0.06 mass % is80 ΔHV. Therefore, by determining whether or not a difference between ameasurement value of the Vickers hardness at the position with the depthof 0.02×T+0.175 (mm) (0.02×T+0.145+0.03 mm) from the surface and themeasurement value of the Vickers hardness at the core portion is notless than 80 ΔHV which is the reference value, it is assured that thenitrogen concentration at the position with the depth of 0.02×T+0.145(mm) from the surface is not less than 0.06 mass % before theaforementioned heat treatment, and as a result, it is assured that thenitrogen concentration in the surface is not less than 0.4 mass %.Therefore, according to the bearing part in accordance with one aspectof the present invention, there can be provided a bearing part which isquantitatively assured to have a higher nitrogen concentration than thatof the conventional bearing part.

In the bearing part, the time of the carbonitriding treatment may be notless than 4 hours and not more than 10 hours. In addition, the time ofthe carbonitriding treatment may be not less than 6 hours and not morethan 8 hours. When the time of the carbonitriding treatment is not lessthan 4 hours and not more than 10 hours, a Vickers hardness at aposition with a depth of not less than 0.26 (mm) and not more than 0.38(mm) from the surface is higher than the Vickers hardness at the coreportion by not less than 80 HV. When the time of the carbonitridingtreatment is not less than 6 hours and not more than 8 hours, a Vickershardness at a position with a depth of not less than 0.3 (mm) and notmore than 0.34 (mm) from the surface is higher than the Vickers hardnessat the core portion by not less than 80 HV.

If the time of the carbonitriding treatment is shorter than 4 hours, thecarbonitrided layer cannot be formed deeply, and thus, it is difficultto set the nitrogen concentration in the surface to be not less than 0.4mass % when a grinding allowance is required after the heat treatment.On the other hand, if the time of the carbonitriding treatment exceeds10 hours, an amount of remaining austenite becomes excessive and thebearing part is likely to have a low hardness, and further, the bearingpart undergoes a significant dimensional change over time. Therefore, itis preferable to select the time of the carbonitriding treatment asappropriate within a range that can assure the durability anddimensional stability of the bearing part (not less than 4 hours and notmore than 10 hours).

A bearing part in accordance with another aspect of the presentinvention is a bearing part made of JIS standard SUJ2 and having acarbonitrided layer formed in a surface thereof. In the bearing part,after heat treatment at a heating temperature of 500° C. for a retentiontime of one hour is performed after carbonitriding treatment for formingthe carbonitrided layer and before grinding processing of the surface, aVickers hardness at a position with a depth of 0.02×T+0.175+t (mm) fromthe surface is higher than a Vickers hardness at a core portion, whichis a region where the carbonitrided layer is not formed in a thicknessdirection of the bearing part, by not less than 80 HV, where Trepresents the time of the carbonitriding treatment and t (mm)represents a thickness of the bearing part removed by the grindingprocessing.

In the bearing part in accordance with another aspect of the presentinvention, by determining whether or not a difference between ameasurement value of a Vickers hardness at a position with a depth wherethe thickness of the bearing part removed by the grinding processing ispreliminarily taken into consideration, i.e., at a position with a depthof 0.02×T+0.175 (mm) plus aforementioned thickness t (mm), and themeasurement value of the Vickers hardness at the core portion is notless than 80 ΔHV, it is possible to assure that the nitrogenconcentration in the surface subjected to the grinding processing is notless than 0.4 mass % even before the grinding processing is performed.Therefore, according to the bearing part in accordance with anotheraspect of the present invention, there can be provided a bearing partwhich is quantitatively assured to have a higher nitrogen concentrationthan that of the conventional bearing part, similarly to theaforementioned bearing part in accordance with one aspect of the presentinvention.

In the bearing part, the time of the carbonitriding treatment may be notless than 4 hours and not more than 10 hours, and the thickness may benot more than 0.125 (mm). In addition, the time of the carbonitridingtreatment may be not less than 6 hours and not more than 8 hours, andthe thickness may be not more than 0.15 (mm).

As described above, by determining the thickness of the bearing partremoved by the grinding processing in accordance with the time of thecarbonitriding treatment, a high nitrogen concentration in the surfacesubjected to the grinding processing can be assured and deformation ofthe bearing part caused by the heat treatment can be corrected.

In the bearing part, the carbonitriding treatment may be performed in atemperature range of not less than 840° C. and not more than 860° C.

If the temperature of the carbonitriding treatment is lower than 840°C., the diffusion speed of nitrogen in steel decreases and the time ofthe carbonitriding treatment becomes longer. On the other hand, if thetemperature of the carbonitriding treatment exceeds 860° C., thedecomposition reaction speed of NH₃ increases and it becomes difficultto keep an undecomposed NH₃ fraction high, and as a result, thetreatment time for assuring a high nitrogen concentration in the surfacesubjected to grinding becomes longer. In addition, a prior austenitecrystal grain size is likely to become coarse. For these reasons, thetemperature range of the carbonitriding treatment is preferably not lessthan 840° C. and not more than 860° C., and more preferably 850° C.

In the bearing part, a prior austenite crystal grain size may be withina range of JIS standard No. 9 to No. 11. The prior austenite crystalgrain size depends on the heating temperature during the carbonitridingtreatment. Therefore, by confirming that the prior austenite crystalgrain size in the bearing part is within the aforementioned range, itcan be confirmed that the carbonitriding treatment is being performed atan appropriate temperature.

A rolling bearing in accordance with the present invention includes thebearing part in accordance with the present invention which isquantitatively assured to have a higher nitrogen concentration ascompared with the conventional bearing part. Therefore, according to therolling bearing in accordance with the present invention, there can beprovided a rolling bearing in which the rolling contact fatigue life isextended and a higher degree of safety is assured as compared with theconventional rolling bearing.

The rolling bearing may be any one of a deep groove ball bearing, aconical roller bearing, a cylindrical roller bearing, and a needleroller bearing. The rolling bearing in accordance with the presentinvention which is assured to have a higher degree of safety is suitableas a rolling bearing such as a deep groove ball bearing, a conicalroller bearing, a cylindrical roller bearing, and a needle rollerbearing.

Advantageous Effects of Invention

As is clear from the foregoing description, according to the bearingpart in accordance with the present invention, there can be provided abearing part which is quantitatively assured to have a higher nitrogenconcentration. In addition, according to the rolling bearing inaccordance with the present invention, there can be provided a rollingbearing which is assured to have a higher degree of safety.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic cross-sectional view showing a structure of a deepgroove ball bearing.

FIG. 2 is a schematic cross-sectional view showing a structure of aconical roller bearing.

FIG. 3 is a schematic cross-sectional view showing a structure of acylindrical roller bearing.

FIG. 4 is a schematic cross-sectional view showing a structure of athrust needle roller bearing.

FIG. 5 is a flowchart schematically showing a method for manufacturing abearing part.

FIG. 6 is a flowchart schematically showing a method for inspecting thebearing part.

FIG. 7 is a schematic view for describing the method for inspecting thebearing part.

FIG. 8 is a flowchart schematically showing another method formanufacturing a bearing part.

FIG. 9 is a graph showing a relationship between a tempering temperatureand a cross sectional hardness difference.

FIG. 10 is a graph showing a relationship between a nitrogenconcentration and a cross sectional hardness difference.

FIG. 11 is a graph showing a relationship between a nitrogenconcentration and a cross sectional hardness difference when thenitrogen concentration is in a range of not more than 0.1 mass %.

FIG. 12 is a graph showing a nitrogen concentration distribution in asteel material after carbonitriding treatment.

FIG. 13 is a graph showing a relationship between the carbonitridingtreatment time and a distance between a depth position with 0.4 mass %and a depth position with 0.06 mass %.

FIG. 14 is a graph showing a relationship between the carbonitridingtreatment time and a distance between a depth position with 0.4 mass %and a depth position with 0.06 mass %.

DESCRIPTION OF EMBODIMENTS

An embodiment of the present invention will be described hereinafterwith reference to the drawings. In the following drawings, the same orcorresponding portions are denoted by the same reference numerals, anddescription thereof will not be repeated.

First, a structure of a deep groove ball bearing 1, which is a rollingbearing according to one embodiment of the present invention, will bedescribed. Referring to FIG. 1, deep groove ball bearing 1 mainlyincludes an outer race 10, an inner race 11, a plurality of balls 12,and a holder 13.

Outer race 10 has an annular shape and has an outer race raceway surface10A on an inner circumferential surface thereof. Inner race 11 has anannular shape and has an inner race raceway surface 11A on an outercircumferential surface thereof. Inner race 11 is arranged on the innerside of outer race 10 such that inner race raceway surface 11A facesouter race raceway surface 10A. Outer race 10 and inner race 11 have anouter diameter of, for example, not more than 150 mm.

By holder 13 made of synthetic resin, balls 12 are aligned at apredetermined pitch on an annular raceway along the circumferentialdirection of outer race 10 and inner race 11, and are held on theraceway in a freely rolling manner. Ball 12 has a ball rolling surface12A and is in contact with outer race raceway surface 10A and inner raceraceway surface 11A at ball rolling surface 12A. With such aconfiguration, outer race 10 and inner race 11 of deep groove ballbearing 1 are relatively rotatable with respect to each other.

Outer race 10, inner race 11 and ball 12 are each a bearing partaccording to the present embodiment made of JIS standard SUJ2 and havinga carbonitrided layer formed in a surface (outer race raceway surface10A, inner race raceway surface 11A and ball rolling surface 12A)thereof. In outer race 10, inner race 11 and ball 12, after heattreatment (hereinafter referred to as “high-temperature tempering”) at aheating temperature of 500° C. for a retention time of one hour isperformed, a Vickers hardness at a position with a depth of 0.02×T+0.175(mm) from the surface is higher than a Vickers hardness at a coreportion, which is a region where the carbonitrided layer is not formedin a thickness direction of the bearing part (outer race 10, inner race11 and ball 12), by not less than 80 HV, where T represents the time ofcarbonitriding treatment for forming the carbonitrided layer. Thisassures that a nitrogen concentration in the aforementioned surface ofthe aforementioned bearing part is not less than 0.4 mass % as describedbelow. As a result, a higher degree of safety can be assured for deepgroove ball bearing 1 including outer race 10, inner race 11 and ball12.

In accordance with the aforementioned relational equation of(0.02×T+0.175), when the time (T) of the carbonitriding treatment is notless than 4 hours and not more than 10 hours, a Vickers hardness at aposition with a depth of not less than 0.26 (mm) and not more than 0.38(mm) from the aforementioned surface is higher than the Vickers hardnessat the aforementioned core portion by not less than 80 HV. When the time(T) of the carbonitriding treatment is not less than 6 hours and notmore than 8 hours, a Vickers hardness at a position with a depth of notless than 0.3 (mm) and not more than 0.34 (mm) from the aforementionedsurface is higher than the Vickers hardness at the aforementioned coreportion by not less than 80 HV.

In addition, in outer race 10, inner race 11 and ball 12, afterhigh-temperature tempering is performed after carbonitriding treatmentfor forming the carbonitrided layer and before grinding processing ofthe surface, a Vickers hardness at a position with a depth of0.02×T+0.175+t (mm) from the surface is higher than a Vickers hardnessat a core portion, which is a region where the carbonitrided layer isnot formed in a thickness direction of the bearing part, by not lessthan 80 HV, where T represents the time of the carbonitriding treatmentand t (mm) represents a thickness of the bearing part removed by thegrinding processing. This can assure that the nitrogen concentration inthe aforementioned surface subjected to the grinding processing is notless than 0.4 mass % even before the grinding processing is performed.

In the aforementioned bearing part, when the time (T) of thecarbonitriding treatment is not less than 4 hours and not more than 10hours, the aforementioned thickness (t) may be not more than 0.125 (mm).When the time (T) of the carbonitriding treatment is not less than 6hours and not more than 8 hours, the aforementioned thickness (t) may benot more than 0.15 (mm).

In addition, in outer race 10 and inner race 11, the thickness(hereinafter referred to as “grinding allowance for one surface”) of theaforementioned bearing part removed by the grinding processing isdetermined in consideration of an amount of deformation and an amount ofdimensional change caused by the heat treatment, and the amount ofdeformation and the amount of dimensional change vary depending on theshape, thickness, cooling conditions and heating conditions of outerrace 10 and inner race 11. Therefore, an average value of the grindingallowance for one surface for outer race raceway surface 10A of outerrace 10 and inner race raceway surface 11A of inner race 11 isdetermined for each model number of the bearing, based on an actualvalue. When outer race 10 and inner race 11 have an outer diameter ofnot more than 150 (mm) as in the present embodiment, the average valueof the grinding allowance for one surface for outer race raceway surface10A and inner race raceway surface 11A is within a range of not lessthan 0.1 mm and not more than 0.2 mm.

As described above, in outer race 10, inner race 11 and ball 12, thetime of the carbonitriding treatment for forming the carbonitrided layeris preferably within the range of not less than 4 hours and not morethan 10 hours, and more preferably within the range of not less than 6hours and not more than 8 hours. If the time of the carbonitridingtreatment is shorter than 4 hours, the carbonitrided layer cannot beformed sufficiently and it is difficult to sufficiently extend therolling contact fatigue life of the bearing part. On the other hand, ifthe time of the carbonitriding treatment exceeds 10 hours, an amount ofremaining austenite becomes excessive and the bearing part undergoes asignificant dimensional change over time. Therefore, from theperspective of providing a bearing part excellent in durability anddimensional stability, it is preferable to set the time of thecarbonitriding treatment to be within the aforementioned range.

In addition, in outer race 10, inner race 11 and ball 12, thetemperature of the carbonitriding treatment for forming thecarbonitrided layer is preferably not less than 840° C. and not morethan 860° C. If the temperature of the carbonitriding treatment is lowerthan 840° C., the diffusion speed of nitrogen in steel decreases, andthus, the time of the carbonitriding treatment needs to be extended. Onthe other hand, if the time of the carbonitriding treatment exceeds 860°C., an amount of entry of nitrogen into steel decreases, and thus, aregion having a high nitrogen concentration is formed to be biasedtoward the surface side of the bearing part. As a result, the time ofthe carbonitriding treatment needs to be extended to increase thenitrogen concentration in the surface. For these reasons, thetemperature of the carbonitriding treatment is preferably not less than840° C. and not more than 860° C., and more preferably 850° C.

In addition, the temperature of the carbonitriding treatment can bedetermined in accordance with a prior austenite crystal grain size afterhardening. For example, when the temperature of the carbonitridingtreatment is 850° C., the prior austenite crystal grain size of theaforementioned bearing part (outer race 10, inner race 11 and ball 12)made of JIS standard SUJ2 is within a range of JIS standard No. 9 to No.11. Therefore, based on the prior austenite crystal grain size afterhardening, it can be confirmed that the carbonitriding treatment hasbeen performed at an appropriate temperature.

In addition, the rolling bearing according to the present invention isnot limited to deep groove ball bearing 1 and may be any one of aconical roller bearing 2 (see FIG. 2), a cylindrical roller bearing 3(see FIG. 3), and a thrust needle roller bearing 4 (see FIG. 4). Thestructure of each of conical roller bearing 2, cylindrical rollerbearing 3 and thrust needle roller bearing 4 will be describedhereinafter.

First, the structure of conical roller bearing 2 will be described withreference to FIG. 2. Conical roller bearing 2 mainly includes an outerrace 20, an inner race 21, a plurality of rollers 22, and a holder 23.Outer race 20 has an annular shape and has an outer race raceway surface20A on an inner circumferential surface thereof. Inner race 21 has anannular shape and has an inner race raceway surface 21A on an outercircumferential surface thereof. Inner race 21 is arranged on the innerside of outer race 20 such that inner race raceway surface 21A facesouter race raceway surface 20A.

Rollers 22 are in contact with inner race raceway surface 21A and outerrace raceway surface 20A, and are circumferentially arranged at apredetermined pitch by holder 23 made of synthetic resin. As a result,rollers 22 are held on an annular raceway of outer race 20 and innerrace 21 in a freely rolling manner. In addition, conical roller bearing2 is configured such that apexes of a cone including outer race racewaysurface 20A, a cone including inner race raceway surface 21A and a coneincluding a path of a rotation axis when rollers 22 roll intersect atone point on a center line of the bearing. With such a configuration,outer race 20 and inner race 21 of conical roller bearing 2 arerelatively rotatable with respect to each other.

Similarly to outer race 10, inner race 11 and ball 12, outer race 20,inner race 21 and roller 22 are each a bearing part according to thepresent embodiment. Therefore, in outer race 20, inner race 21 androller 22, after high-temperature tempering is performed, a Vickershardness at a position with a depth of 0.02×T+0.175 (mm) from thesurface (outer race raceway surface 20A, inner race raceway surface 21and a roller rolling surface 22A) is higher than a Vickers hardness at acore portion, which is a region where the carbonitrided layer is notformed in a thickness direction of the bearing part (outer race 20,inner race 21 and roller 22), by not less than 80 HV, where T representsthe time of carbonitriding treatment for forming the carbonitridedlayer. This assures that a nitrogen concentration in the aforementionedsurface of outer race 20, inner race 21 and roller 22 is not less than0.4 mass %. As a result, a higher degree of safety is assured forconical roller bearing 2 as well.

Next, the structure of cylindrical roller bearing 3 will be describedwith reference to FIG. 3. Cylindrical roller bearing 3 mainly includesan outer race 30, an inner race 31, a plurality of rollers 32, and aholder 33. Outer race 30 has an annular shape and has an outer raceraceway surface 30A formed on an inner circumferential surface thereof.Inner race 31 has an annular shape and has an inner race raceway surface31A formed on an outer circumferential surface thereof. Inner race 31 isarranged on the inner side of outer race 30 such that inner race racewaysurface 31A faces outer race raceway surface 30A.

Roller 32 has a cylindrical shape and is in contact with inner raceraceway surface 31A and outer race raceway surface 30A at a rollerrolling surface 32A. In addition, rollers 32 are circumferentiallyarranged at a predetermined pitch by holder 33 made of synthetic resin,and thus, are held on an annular raceway of outer race 30 and inner race31 in a freely rolling manner. With such a configuration, outer race 30and inner race 31 of cylindrical roller bearing 3 are relativelyrotatable with respect to each other.

Similarly to outer race 10, inner race 11 and ball 12, outer race 30,inner race 31 and roller 32 are each a bearing part according to thepresent embodiment. Therefore, in outer race 30, inner race 31 androller 32, after high-temperature tempering is performed, a Vickershardness at a position with a depth of 0.02×T+0.175 (mm) from thesurface (outer race raceway surface 30A, inner race raceway surface 31Aand roller rolling surface 32A) is higher than a Vickers hardness at acore portion, which is a region where the carbonitrided layer is notformed in a thickness direction of the bearing part (outer race 30,inner race 31 and roller 32), by not less than 80 HV, where T representsthe time of carbonitriding treatment for forming the carbonitridedlayer. This assures that a nitrogen concentration in the aforementionedsurface of outer race 30, inner race 31 and roller 32 is not less than0.4 mass %. As a result, a higher degree of safety is assured forcylindrical roller bearing 3 as well.

Next, the structure of thrust needle roller bearing 4 will be describedwith reference to FIG. 4. Thrust needle roller bearing 4 mainly includesa pair of bearing rings 40 and 41, a plurality of needle rollers 42 andan annular holder 43.

Bearing rings 40 and 41 have a shape of a circular disk and are arrangedsuch that a main surface of one bearing ring faces a main surface of theother bearing ring. At a roller rolling surface 42A which is an outercircumferential surface of needle roller 42, needle roller 42 is incontact with bearing ring raceway surfaces 40A and 41A formed on onemain surfaces of the pair of bearing rings 40 and 41 that face eachother. In addition, needle rollers 42 are circumferentially arranged ata predetermined pitch by holder 43, and thus, are held on an annularraceway in a freely rolling manner. With such a configuration, the pairof bearing rings 40 and 41 of the thrust needle roller bearing arerelatively rotatable with respect to each other.

Similarly to outer race 10, inner race 11 and ball 12, bearing rings 40and 41 and needle roller 42 are each a bearing part according to thepresent embodiment. Therefore, in bearing rings 40 and 41 and needleroller 42, after high-temperature tempering is performed, a Vickershardness at a position with a depth of 0.02×T+0.175 (mm) from thesurface (bearing ring raceway surfaces 40A and 41A and roller rollingsurface 42A) is higher than a Vickers hardness at a core portion, whichis a region where the carbonitrided layer is not formed in a thicknessdirection of the bearing part (bearing rings 40 and 41 and needle roller42), by not less than 80 HV, where T represents the time ofcarbonitriding treatment for forming the carbonitrided layer. Thisassures that a nitrogen concentration in the aforementioned surface ofbearing rings 40 and 41 and needle roller 42 is not less than 0.4 mass%. As a result, a higher degree of safety is assured for thrust needleroller bearing 4 as well.

Next, a method for inspecting the bearing part according to the presentembodiment will be described. In the method for inspecting the bearingpart according to the present embodiment, it is possible to assure thatthe nitrogen concentration in the surface (outer race raceway surface10A, inner race raceway surface 11A and ball rolling surface 12A) ofouter race 10, inner race 11 and ball 12 is not less than 0.4 mass %.

In addition, the method for inspecting the bearing part according to thepresent embodiment is performed in a method for manufacturing thebearing part such as outer race 10, inner race 11 and ball 12. Morespecifically, referring to FIG. 5, the aforementioned method formanufacturing the bearing part includes the steps of: preparing a steelmaterial (S10); molding the steel material (S20); subjecting the moldedbody to carbonitriding treatment (S30); subjecting the molded body tohardening treatment (S40); subjecting the molded body to temperingtreatment (S50); subjecting the molded body to grinding processing(S60); and inspection (S70), and the method for inspecting the bearingpart according to the present embodiment is performed in the step (S70).In the present specification, the step (S70) will be described in detailand the detailed description of the steps (S10) to (S60) will not beprovided.

Referring to FIG. 6, in the method for inspecting the bearing partaccording to the present embodiment, a step of determining a positionfor measuring a cross sectional hardness is first performed as a step(S71). In this step (S71), by substituting the time (T) of thecarbonitriding treatment into the equation of 0.02×T+0.175, the positionwith a depth for measuring the cross sectional hardness (Vickershardness) in a step (S73) described below is calculated.

Next, a step of high-temperature tempering is performed as a step (S72).In this step (S72), the bearing part such as inner race 11 (see FIG. 1)is prepared and heat treatment is performed on the bearing part underconditions of a heating temperature of not less than 300° C. and notmore than 700° C. and a retention time of one hour. As a result, aVickers hardness at a carbonitrided layer in inner race 11 becomeshigher than a Vickers hardness at a core portion, which is a regionwhere the carbonitrided layer is not formed. The heating temperature ismore preferably not less than 400° C. and not more than 600° C., andfurther preferably 500° C.

Next, a step of measuring the cross sectional hardness is performed as astep (S73). In this step (S73), referring to FIG. 7, a test piece 14 isfirst cut out of inner race 11. Then, the Vickers hardness at theposition with the depth calculated in the aforementioned step (S71) andthe Vickers hardness at the aforementioned core portion are measured,respectively, from an outer circumferential surface 14A of test piece14.

Next, a step of calculating a difference in cross sectional hardness isperformed as a step (S74). In this step (S74), a difference (hereinafterreferred to as “cross sectional hardness difference”) between theVickers hardness at the position with the depth calculated in theaforementioned step (S71) and the Vickers hardness at the aforementionedcore portion is calculated from outer circumferential surface 14A.

Next, a step of comparing the cross sectional hardness difference with areference value is performed as a step (S75). In this step (S75), thevalue of the cross sectional hardness difference calculated in theaforementioned step (S74) is compared with 80 ΔHV which is thepredetermined reference value, and it is determined whether or not thecross sectional hardness difference is not less than 80 ΔHV. When thecross sectional hardness difference is not less than 80 ΔHV, it ispossible to assure that the nitrogen concentration in the surface ofinner race 11 before high-temperature tempering is not less than 0.4mass % as described below. As described above, according to the methodfor inspecting the bearing part in accordance with the presentembodiment, a high nitrogen concentration in the surface of the bearingpart can be quantitatively assured.

Referring to FIG. 8, in the aforementioned method for manufacturing thebearing part, the step of inspection (S70) may not only be performedafter the step of grinding processing (S60) but also be performed afterthe step of tempering treatment (S50). In this case, in the step ofdetermining the position for measuring the cross sectional hardness(S71) (see FIG. 6), the position with the depth for measuring theVickers hardness in the step (S73) described below is calculated bysubstituting the time (T) of the carbonitriding treatment and athickness (t) of the bearing part removed by grinding processing intothe equation of 0.02×T+0.175+t. As described above, the position formeasuring the cross sectional hardness is determined with considerationpreliminarily given to the thickness of the bearing part removed bygrinding processing, and thus, it is possible to assure that thenitrogen concentration in the surface subjected to grinding processingis not less than 0.4 mass % even before the bearing part is subjected togrinding processing.

EXAMPLE

In the present example, description will be provided of a method fordetermining the position for measuring the cross sectional hardness, thereference value of the cross sectional hardness difference and the liketo assure that the nitrogen concentration in the surface of the bearingpart is not less than 0.4 mass %.

(1) Test Piece and Experimental Method

(1-1) Introduction

Firstly, it is necessary to determine the heating temperature and theretention time in the high-temperature tempering which have a highcorrelation with the nitrogen concentration. Since transformation by thehigh-temperature tempering is a thermally-activated process, increasingthe heating temperature and increasing the retention time have the samemeaning, and it is considered meaningless to use the both as variables.Thus, in the present example, the optimum heating temperature wasdetermined, by setting the retention time to a fixed time (one hour) andchanging the heating temperature to 300° C., 400° C., 500° C., 600° C.,and 700° C. to investigate a heating temperature at which a differencein hardness was clearest.

Further, it is considered that a difference in hardenability and adifference in cooling rate during hardening due to a difference inchemical components in each material may affect a hardness after thehardening and also may affect a hardness after the high-temperaturetempering. Thus, in the present example, the absolute value itself of across sectional hardness is not used, but a hardness difference betweena hardness at a non-nitrided position deep from a surface layer (coreportion) (here, for example, a hardness at a depth of 1 mm from anuppermost surface subjected to heat treatment) and a hardness at aposition with an arbitrary depth within a nitrided region was adopted asan indicator. Namely, the chemical components in each material may varyfrom material lot to material lot, and the hardness difference is foroffsetting such a difference.

(1-2) Target Test Pieces

Table 1 shows chemical components of test pieces subjected to aninvestigation. The materials were all made of JIS standard SUJ2, andsubjected to carbonitriding treatment in various heat treatment furnacesand under various heat treatment atmospheres. It is noted that thecarbonitriding treatment temperature was included in a temperature rangeof not less than 840° C. and not more than 860° C.

TABLE 1 Test Piece Chemical Components (mass %) No. C Si Mn Ni Cr Mo CuO* 1 1.03 0.25 0.35 0.01 1.50 0 0.01 4 2 unknown 3 0.98 0.27 0.47 0.051.45 0.02 0.10 6 4 1.00 0.27 0.35 0.02 1.51 0 0.01 7 5 1.01 0.24 0.350.01 1.49 0 0.01 3 6 0.99 0.26 0.37 0.02 1.44 0 0.01 5 7 unknown 8 0.980.25 0.34 0.07 1.43 0.03 0.10 6 9 0.98 0.25 0.34 0.07 1.43 0.03 0.10 610 0.98 0.25 0.34 0.07 1.43 0.03 0.10 6 *ppm

Specifically, test piece No. 1 was subjected to treatment underconditions of a carbonitriding treatment temperature of 850° C., atreatment time of 120 minutes (min.) (hereinafter expressed as “850°C.×120 min”), an undecomposed ammonia fraction of 0.2 vol. %, and acarbon activity of 0.9. Test piece No. 2 was subjected to treatmentunder conditions of 840° C.×70 min., an undecomposed ammonia fraction of0.1 vol. %, and a carbon activity of 0.85. Test piece No. 3 wassubjected to treatment under conditions of 850° C.×120 min., anundecomposed ammonia fraction of 0.1 vol. %, and a carbon activity of0.9. Test piece No. 4 was subjected to treatment under conditions of850° C.×90 min., an undecomposed ammonia fraction of 0.1 vol. %, and acarbon activity of 0.9. Test piece No. 5 was subjected to treatmentunder conditions of 850° C.×90 min., an undecomposed ammonia fraction of0.1 vol. %, and a carbon activity of 0.9.

In addition, test piece No. 6 was subjected to treatment underconditions of 850° C.×90 min., an undecomposed ammonia fraction of 0.13vol. %, and a carbon activity of 0.9. Test piece No. 7 was subjected totreatment under conditions of 850° C.×150 min., an undecomposed ammoniafraction of 0.1 vol. %, and a carbon activity of 0.85. Test piece No. 8was subjected to treatment under conditions of 850° C.×150 min., anundecomposed ammonia fraction of 0.25 vol. %, and a carbon activity of0.9. Test piece No. 9 was subjected to treatment under conditions of850° C.×180 min., an undecomposed ammonia fraction of 0.3 vol. %, and acarbon activity of 0.95. Test piece No. 10 was subjected to treatmentunder conditions of 850° C.×90 min., an undecomposed ammonia fraction of0.2 vol. %, and a carbon activity of 0.9.

(1-3) Method for Measuring Nitrogen Concentration

In order to investigate correlation between a cross sectional hardnessand a nitrogen concentration of a sample subjected to thehigh-temperature tempering, it is necessary to measure nitrogenconcentration distribution in the sample (steel). Line analysis with anEPMA was used to measure the nitrogen concentration in the steelsubjected to the carbonitriding treatment. Quantification was performedby analyzing a calibration test piece having a known nitrogenconcentration and using a calibration curve thereof. A schematic view ofa sample used for EPMA analysis and a measurement method is as shown inFIG. 7.

It is assumed that, for example, inner race 11 (see FIG. 1) is used as asample as shown in FIG. 7. For the sample, a nitrogen concentration inthe sample subjected to the carbonitriding treatment was measured.Specifically, test piece 14 as shown in FIG. 7 was cut out of thesample, and line analysis with an EPMA was performed on a cut-out endsurface at a central portion in a height direction of test piece 14(i.e., at a position with a half width) along a direction from outercircumferential surface 14A to inner circumferential surface 14B of thetest piece.

(1-4) Method for Measuring Cross Sectional Hardness

Hardness was measured at the cut-out end surface subjected to the EPMAanalysis in test piece 14 described above in (1-3). As a measuringmethod, Vickers hardness measurement was performed using a micro Vickershardness tester.

(2) Search for Retention Temperature for High-Temperature Tempering

(2-1) Experiment Description

In order to search for a tempering temperature (heating temperature)having a high correlation with the nitrogen concentration, test pieces14 subjected to the carbonitriding treatment were subjected to temperingat a heating temperature of 180° C. for a retention time of two hours,and thereafter subjected to five types of high-temperature tempering atheating temperatures of 300° C., 400° C., 500° C., 600° C., and 700° C.for a retention time of one hour. The high-temperature tempering wasperformed in an air atmosphere. Then, cross sectional hardnesses of thetest pieces treated under the respective conditions for thehigh-temperature tempering were measured. Here, measurement wasperformed on test pieces No. 8 and No. 9 which were under carbonitridingtreatment conditions considered to cause a large amount of nitrogen toenter the test pieces.

(2-2) Experimental Results

FIG. 9 shows a graph compiling experimental results. The graph shown inFIG. 9 compiles the experimental results, with the tempering temperatureon the axis of abscissas and the difference in cross sectional hardness(i.e., [the maximum value of the cross sectional hardness]−[the crosssectional hardness at a position with the depth of 1 mm from theuppermost surface subjected to the heat treatment]: also expressed asΔHV) on the axis of ordinates. As can be seen from FIG. 9, differenceΔHV in cross sectional hardness was maximum after the high-temperaturetempering at the heating temperature of 500° C. for the retention timeof one hour. Difference ΔHV in cross sectional hardness at the heatingtemperature of 500° C. had a value about double that of difference ΔHVin cross sectional hardness after the high-temperature tempering at theheating temperature of 300° C. or 700° C. Accordingly, it is consideredthat the hardness after the tempering having a relatively highcorrelation with the nitrogen concentration is the hardness after thetempering at a heating temperature of about 500° C. Therefore, in anexperiment below, measurement of the cross sectional hardness wasperformed on the test pieces subjected to high-temperature tempering ata heating temperature of 500° C. for a retention time of one hour.

(3) Investigation of Relationship between Nitrogen Concentration andDifference (ΔHV) in Cross Sectional Hardness

Here, each test piece having a composition shown in Table 1 wassubjected to the carbonitriding treatment, and further, the heattreatment at the heating temperature of 500° C. for the retention timeof one hour as the high-temperature tempering, and thereafter a nitrogenconcentration in each test piece 14 was measured by the EPMA analysis asdescribed with reference to FIG. 7. The carbonitriding treatment wasperformed under conditions of a heating temperature of 850° C. and avalue (γ) of carbon activity/undecomposed ammonia fraction of 4.75.Further, a cross sectional hardness in a depth direction of test piece14 was measured in the cut-out end surface shown in FIG. 7. Then,relationship of a difference between a cross sectional hardness at acertain position in the depth direction and the cross sectional hardnessat the position with the depth of 1 (mm) from the uppermost surfacesubjected to the heat treatment (a cross sectional hardness difference(ΔHV)) was investigated. FIGS. 10 and 11 show results thereof.

In FIGS. 10 and 11, the axis of abscissas represents the nitrogenconcentration (unit: mass %), and the axis of ordinates represents thecross sectional hardness difference (ΔHV) (unit: Vickers hardness). FIG.11 shows an excerpt, from FIG. 10, of a relationship between a nitrogenconcentration and a cross sectional hardness difference when thenitrogen concentration is in a range of 0 to 0.1 mass %. It has beenfound from FIG. 10 that, when the nitrogen concentration is in a rangeof more than 0 and not more than 0.1 mass %, the correlation between thenitrogen concentration and the cross sectional hardness difference isstrong, whereas when the nitrogen concentration is in a range of morethan 0.1 mass %, the correlation between the nitrogen concentration andthe cross sectional hardness difference is relatively weak. This isconsidered to be because incomplete hardening may occur in a region witha high nitrogen concentration, and nitrogen dissolved in the test piecedoes not necessarily contribute to a reduction in the decomposition rateof martensite. In addition, based on FIG. 11, when a correlationcoefficient between the nitrogen concentration and the cross sectionalhardness difference is calculated when the nitrogen concentration is inthe range of more than 0 and not more than 0.1 mass %, a highcorrelation coefficient of 0.8348 is obtained therebetween. Accordingly,it is considered that the nitrogen concentration can be predicted fromthe cross sectional hardness difference if the nitrogen concentration isin the range of 0 to 0.1 mass %. Therefore, in an experiment below, arelationship between a nitrogen concentration of 0.06 mass % and a crosssectional hardness difference of 80 ΔHV, which is a substantiallyintermediate position in a region (0 to 0.1 mass %) where the nitrogenconcentration and the cross sectional hardness difference have apositive correlation, was used.

(4) Relationship Between Carbonitriding Treatment Time and NitrogenConcentration Distribution

In the case of the component of the rolling bearing, grinding processingis performed after hardening and tempering in order to adjust the shapethereof. Therefore, when the carbonitriding treatment is performed undercertain treatment conditions, the nitrogen concentration in the productuppermost surface (raceway surface or rolling surface) changes due to anallowance of the grinding processing. Thus, in order to keep thenitrogen concentration in the product uppermost surface at not less than0.4 mass %, it is necessary to change the conditions for thecarbonitriding treatment in accordance with a grinding allowance for onesurface.

FIG. 12 shows a nitrogen concentration distribution (before thehigh-temperature tempering is performed) when a material of JIS standardSUJ2 is subjected to the carbonitriding treatment for treatment times of4 hours ((A) in the figure), 6 hours ((B) in the figure), 8 hours ((C)in the figure), and 10 hours ((D) in the figure) under conditions of aheating temperature of 850° C. and a value (γ) of carbonactivity/undecomposed ammonia fraction of 4.75. In FIG. 12, the axis ofabscissas represents the depth (mm) from the surface of the SUJ2material, and the axis of ordinates represents the nitrogenconcentration (mass %). Here, when the carbonitriding treatment isperformed under a condition of the γ value larger than 5, the amount ofnitrogen entry decreases and the region having a high nitrogenconcentration is further biased toward the surface side. As a result, itbecomes practically difficult to set the nitrogen concentration in theproduct surface at not less than 0.4 mass % by using a method other thana method for significantly increasing the treatment time. In addition,when the temperature of the carbonitriding treatment is not less than860° C., it is difficult to keep the γ value at not more than 5. Whenthe temperature of the carbonitriding treatment is not more than 840°C., the diffusion speed of nitrogen into the steel decreases, and as aresult, the treatment time becomes longer. Therefore, a temperature ofabout 850° C. is appropriate for the carbonitriding treatment of theSUJ2 material. It should be noted that the heating temperature at thetime of the carbonitriding treatment can be determined by the size ofthe prior austenite crystal grain after hardening, and when the heatingtemperature is 850° C., the prior austenite crystal grain size is withina range of JIS standard No. 9 to No. 11 for the SUJ2 material.

It has been found from FIG. 12 that the nitrogen concentration in theproduct uppermost surface can be set at not less than 0.4 mass %, bysetting the carbonitriding treatment time at 4 hours when the grindingallowance for one surface in the product uppermost surface is 0.125(mm), by setting the carbonitriding treatment time at 6 hours when thegrinding allowance for one surface in the product uppermost surface is0.15 (mm), by setting the carbonitriding treatment time at 8 hours whenthe grinding allowance for one surface in the product uppermost surfaceis 0.175 (mm), and by setting the carbonitriding treatment time at 10hours when the grinding allowance for one surface in the productuppermost surface is 0.2 (mm).

(5) Appropriate Position for Measuring Cross Sectional HardnessDifference

A depth with 0.4 mass % and a depth with 0.06 mass % in eachcarbonitriding treatment time are obtained from FIG. 12, and a distancetherebetween is plotted on the axis of ordinates and the treatment timeis plotted on the axis of abscissas. Then, the graph shown in FIG. 13 isobtained. As described above, in order to assure that the nitrogenconcentration in the product uppermost surface is not less than 0.04mass % by using a relationship that the cross sectional hardnessdifference of 80 ΔHV corresponds to the nitrogen concentration of 0.06mass %, it is necessary to investigate a cross sectional hardnessdifference at a position with a depth of not less than the distanceshown by the axis of ordinates in FIG. 12.

In addition, as described above, when the high-temperature tempering isperformed at a heating temperature of 500° C. for a retention time ofone hour, nitrogen in the steel is diffused into the inside by about0.03 (mm). Thus, when 0.03 (mm) is added to the axis of ordinates inFIG. 13, an appropriate position for measuring the cross sectionalhardness difference in each treatment time becomes a position with adepth shown by a value on the axis of ordinates in FIG. 14, i.e., aposition with a depth of 0.02×T+0.175 (mm) (T: the carbonitridingtreatment time). Therefore, if the cross sectional hardness differenceat the position with the depth of 0.02×T+0.175 (mm) from the surfacesubjected to the high-temperature tempering is not less than 80 ΔHV, itis possible to assure that the nitrogen concentration in the surfacebefore the high-temperature tempering is not less than 0.4 mass %.

(6) Grinding Allowance for One Surface in Raceway Surfaces of Inner Raceand Outer Race of Rolling Bearing

When the inner race and the outer race of the rolling bearing arehardened, deformation and dimensional change caused by heat treatmentoccur. The amount of this deformation and the amount of this dimensionalchange vary depending on the shape, thickness, cooling conditions,heating conditions and the like of the product. Therefore, an averageamount of the grinding allowance for one surface in the inner race andthe outer race is generally determined for each model number of therolling bearing, by using an actual value as a reference. In the rollingbearing in which the inner race and the outer race have an outerdiameter of not more than 150 (mm), an average value of the grindingallowance for one surface in the raceway surfaces is 0.1 to 0.2 (mm) inmany cases. Therefore, if the nitrogen concentration distribution shownin FIG. 12 and the position for measuring the cross sectional hardnessdifference shown in FIG. 14 are clear, they may be sufficient forassuring that the nitrogen concentration in the raceway surfaces of theinner race and the outer race of the rolling bearing having an outershape of not more than 150 (mm) is not less than 0.4 mass %.

(7) Procedure of Quality Assurance

(7-1) Procedure of Quality Assurance After Heat Treatment

Based on the examination results described above in (1) to (6), aprocedure for assuring that the nitrogen concentration in the productuppermost surface subjected to the heat treatment (after thecarbonitriding treatment, the hardening treatment and the temperingtreatment and before the grinding processing) is not less than 0.4 mass% is as follows. First, the product dimension after the carbonitridingtreatment, hardening and tempering is measured and an average value (t)of the grinding allowance for one surface in the raceway surfaces iscalculated. Next, the time (T) of the carbonitriding treatment and theaverage value (t) of the grinding allowance for one surface aresubstituted into the equation of 0.02×T+0.175+t and the position formeasuring the cross sectional hardness difference after thehigh-temperature tempering is calculated. Next, the high-temperaturetempering (additional tempering) is performed at a heating temperatureof 500° C. for a retention time of one hour. Next, a test piece is cutout of the product subjected to the high-temperature tempering, and thecross sectional hardness difference at the measurement positioncalculated at the test piece is measured. Then, it is determined whetheror not the value of the measured cross sectional hardness difference isnot less than 80 ΔHV which is the reference value. As a result, if thisvalue of the cross sectional hardness difference is not less than 80ΔHV, it is possible to assure that the nitrogen concentration in theproduct uppermost surface is not less than 0.4 mass %.

(7-2) Procedure of Quality Assurance of Finished Product

Based on the examination results described above in (1) to (6), aprocedure for assuring that the nitrogen concentration in the productuppermost surface after completion of the product (after the grindingprocessing) is not less than 0.4 mass % is as follows. First, thetreatment time (T) of the carbonitriding treatment is substituted intothe equation of 0.02×T+0.175 and the position for measuring the crosssectional hardness difference after the high-temperature tempering iscalculated. Next, the high-temperature tempering is performed at aheating temperature of 500° C. for a retention time of one hour. Next, atest piece is cut out of the product subjected to the high-temperaturetempering, and the cross sectional hardness difference at themeasurement position calculated at the test piece is measured. Then, itis determined whether or not the value of the measured cross sectionalhardness difference is not less than 80 ΔHV which is the referencevalue. As a result, if this value of the cross sectional hardnessdifference is not less than 80 ΔHV, it is possible to assure that thenitrogen concentration in the product uppermost surface is not less than0.4 mass %.

It should be understood that the embodiment and the example disclosedherein are illustrative and not limitative in any respect. The scope ofthe present invention is defined by the terms of the claims, rather thanthe description above, and is intended to include any modificationswithin the scope and meaning equivalent to the terms of the claims.

INDUSTRIAL APPLICABILITY

The bearing part and the rolling bearing according to the presentinvention can be particularly advantageously applied to a bearing parthaving a carbonitrided layer formed in a surface thereof, and a rollingbearing including the bearing part.

REFERENCE SIGNS LIST

1 deep groove ball bearing; 2 conical roller bearing; 3 cylindricalroller bearing; 4 needle roller bearing; 10, 20, 30, 40 outer race; 10A,20A, 30A, 40A outer race raceway surface; 11, 21, 31, 41 inner race;11A, 21A, 31A, 41A inner race raceway surface; 12 ball; 12A ball rollingsurface; 13, 23, 33, 43 holder; 14 test piece; 14A outer circumferentialsurface; 14B inner circumferential surface; 22, 32 roller; 22A, 32A, 42Aroller rolling surface; 40, 41 bearing ring; 40A, 41A bearing ringraceway surface; 42 needle roller.

1. A bearing part made of JIS standard SUJ2 and having a carbonitridedlayer formed in a surface thereof, wherein, after heat treatment at aheating temperature of 500° C. for a retention time of one hour isperformed, a Vickers hardness at a position with a depth of 0.02×T+0.175(mm) from said surface is higher than a Vickers hardness at a coreportion, which is a region where said carbonitrided layer is not formedin a thickness direction of said bearing part, by not less than 80 HV,where T represents the time of carbonitriding treatment for forming saidcarbonitrided layer.
 2. The bearing part according to claim 1, whereinthe time of said carbonitriding treatment is not less than 4 hours andnot more than 10 hours.
 3. The bearing part according to claim 1,wherein the time of said carbonitriding treatment is not less than 6hours and not more than 8 hours.
 4. A bearing part made of JIS standardSUJ2 and having a carbonitrided layer formed in a surface thereof,wherein, after heat treatment at a heating temperature of 500° C. for aretention time of one hour is performed after carbonitriding treatmentfor forming said carbonitrided layer and before grinding processing ofsaid surface, a Vickers hardness at a position with a depth of0.02×T+0.175+t (mm) from said surface is higher than a Vickers hardnessat a core portion, which is a region where said carbonitrided layer isnot formed in a thickness direction of said bearing part, by not lessthan 80 HV, where T represents the time of said carbonitriding treatmentand t (mm) represents a thickness of said bearing part removed by saidgrinding processing.
 5. The bearing part according to claim 4, whereinthe time of said carbonitriding treatment is not less than 4 hours andnot more than 10 hours, and said thickness is not more than 0.125 (mm).6. The bearing part according to claim 4, wherein the time of saidcarbonitriding treatment is not less than 6 hours and not more than 8hours, and said thickness is not more than 0.15 (mm).
 7. The bearingpart according to claim 1, wherein said carbonitriding treatment isperformed in a temperature range of not less than 840° C. and not morethan 860° C.
 8. The bearing part according to claim 1, wherein a prioraustenite crystal grain size is within a range of JIS standard No. 9 toNo.
 11. 9. A rolling bearing comprising the bearing part as recited inclaim
 1. 10. The rolling bearing according to claim 9, wherein therolling bearing is any one of a deep groove ball bearing, a conicalroller bearing, a cylindrical roller bearing, and a needle rollerbearing.